Author Topic: CB750A Barn Build (Read 17027 times)

TurboD · « **Reply #25 on:** June 14, 2015, 05:42:28 PM »

I got my cruzin image 836 pistons Friday, and managed to get the engine pretty much together and running today.

BPellerine · « **Reply #26 on:** June 14, 2015, 06:45:08 PM »

you make it look easy!bill

TurboD · « **Reply #27 on:** June 16, 2015, 01:36:00 PM »

Thanks Bill, It is easy. Lol

I have the bike mostly complete, just a few loose ends and some tuning.

BPellerine · « **Reply #28 on:** June 16, 2015, 06:41:32 PM »

I am thinking you drive these bikes on the street as well?how is the bike to get around on going 6ins over on my streetbike.bill

TurboD · « **Reply #29 on:** July 01, 2015, 07:04:26 PM »

As a last minute idea, and totally unprepared I decided to take the Barn Build Auto to the track today.

The bike ran good with the exception of the carbs being lean (which I pretty much expected) I ran it a total of 4 passes before packing it in for the night, a couple of the pipes were starting to color from the lean condition.

The lean condition was causing a flat spot when launching with a slight miss, and then a slight miss again in the midrange around 5k to 6k for a second. Both killing and chance of a good 60 foot time. I was hoping to run 13's the first time out, but It ran a best of 14.08 at 103.5 mph with a slow 2.4 sixty. I expect with the jetting corrected it should really wake up.

Jerry Rxman Griffin aka MuthaF'er · « **Reply #30 on:** July 01, 2015, 08:08:57 PM »

Yes! It lives. Gotta start somewhere huh. The other rider looks a little antsy waiting on the tree while you're gone.

Sam Green Racing · « **Reply #31 on:** July 02, 2015, 06:14:30 AM »

Jerry, I think the other guy is holding back to see the outcome of falling off with just jeans on.
I'm not sure why people bother with helmets to be honest. $:-\$

Sam.

TurboD · « **Reply #32 on:** July 02, 2015, 12:41:50 PM »

In my rush to leave out, my leather pants were one of the things I forgot.

Never hurts to error on the side of safety, but not sure I would equate jeans vs leathers to not having a helmet..

Its good to know that forgetting my pants overshadows the intent of this build thread.

Here's one. I cannot count the number of 8.60 @160 passes I made on this one back in the 90's.

Bill/BentON Racing · « **Reply #33 on:** July 02, 2015, 01:30:56 PM »

Good job, thought Sammy's reply would bring a rebuttal. Keep up the build thread, glad you got leather pants and use them normally.

As someone who tasted asphalt twice in 59 days, it has no mercy, all y'all try and wear the best protection you can. Now the 13's will be next, then the 12 's! K, Bill

dragracer · « **Reply #34 on:** July 02, 2015, 01:45:11 PM »

8.60's, no bar. My kind of racing!!! Turbo for you, nitrous for me.

Good runs for the first time out on the new build. I know you'll get it tweaked before long. The question remains, when will you come race with the Team Sam Green Racing crew???

Sam Green Racing · « **Reply #35 on:** July 02, 2015, 01:57:23 PM »

Quote from: TurboD on July 02, 2015, 12:41:50 PM

In my rush to leave out, my leather pants were one of the things I forgot.

Never hurts to error on the side of safety, but not sure I would equate jeans vs leathers to not having a helmet..

Its good to know that forgetting my pants overshadows the intent of this build thread.

Here's one. I cannot count the number of 8.60 @160 passes I made on this one back in the 90's.

I didn't mean to be disrespectful but I would have thought that knowing what happened to me in full top of the range gear would have been a lesson to all on here. You can loose enough blood from your legs to kill you in minutes if not properly protected, the choice is yours.

Sam.

dragracer · « **Reply #36 on:** July 02, 2015, 03:14:42 PM »

Sam,
I read into your post an air of concern over criticism because I know of your fate. Each time before I make a pass, i make sure I've got my full gear on, gloves sninched, boots tight and helmet snugged. I used to get comfortable on the slower bikes and not take safety as seriously. You showed us all that even low speed crashes can result in debilitating injuries.

Dave has been great in sharing his projects with and we all appreciate his openness. I know I've picked up a trick or two from him.

POPS 911 · « **Reply #37 on:** July 02, 2015, 04:17:22 PM »

Lean Carbs BLUE PIPES : That's a good reason to plumb in 4- dial-a-jets W/CV carbs and with a thin flat blade screw driver in 4 minutes you could have tuned for the lean with 4 more jet options. I bought all the DIAL-A-JET KITS that DIXIE CO. had to sell and have sold many to some of the tinkers than I know....... PS. Those dial-a-jets were developed by a graduate engineer from " THE OHIO STATE UNIVERSITY " another winner in the game in 100 yards not 1320'. He invented these in the late 1980's when the government made all motorcycles coming from the factory with lean fixed jets for the CARB-EPA standard. My brand New 1982 YAMAHA XV920R [ chain drive ] ran great all across the USA on a 30 day trip out West at all elevations.

POPS BK911

Desert-SOHC · « **Reply #38 on:** July 02, 2015, 11:02:08 PM »

What did you do with that bucket seat?

TurboD · « **Reply #39 on:** July 03, 2015, 05:57:01 AM »

Bill I'm happy with the first outing, I knew that it might be a little lean. It had a slight bog and miss right from the hit for a brief second, then ran strong to 5000 where it misses a little until 7000 then clears again and pulls good to 11,000. Yesterday I drilled the jets the next size bigger, this helped the bog and miss at the hit, but did not really change the mid rpm miss. I am going to go another size larger as it is not showing any signs of being rich. I really look for it to go deep in the 13's once I clear this miss up.

Frank I would really like to make one of the races this year with all the guys, I just wish they were closer.

Sam No problem, we never can be too safe. I hope you are doing better and possibly see you later this year.

Pops I would love to have one of those dial a jets on there right now, it would sure help to answer my jetting questions faster.

Desert I still have the bucket seat along with many of the parts I did not use.

SBR-CB400F · « **Reply #40 on:** July 03, 2015, 06:16:35 AM »

I like the barn build. Now get out and race the thing

dragracer · « **Reply #41 on:** July 03, 2015, 07:09:04 AM »

Would raising the needles up help with the mid range problem?? Did that with a set of PD's and it worked to get rid of a mid range stutter.

TurboD · « **Reply #42 on:** July 03, 2015, 08:45:01 AM »

Well Frank I'm a little baffled and in the rethinking mode. This is my first time working with this style Yamaha Carbs. I started out this morning by drilling the mains another size because all the indicators pointed to more jet, however after doing so the bike still runs good everywhere except the now 4800 to 7200 range. Which now I know its too rich in this area, and not lean.

This said I removed the top covers for the first time, I am always reluctant of disturbing the used diaphragms if not totaling needed. I did so in hopes the needles may have been too high causing a rich midrange. They are not adjustable and have springs on them inside of a chamber, and there are 2 more air jets within the diaphragm chamber. Very strange setup.

dragracer · « **Reply #43 on:** July 03, 2015, 02:05:37 PM »

Wow, can you put small washers under the clips to raise them?? I think you're to lean

TurboD · « **Reply #44 on:** July 03, 2015, 02:45:50 PM »

Frank They have a molded on "head" with a spring attached to push down on them, from what I understand they are a "power valve" of sorts. The needle is able to move automatically to adjust.

However I have fixed the problem. I had another set of carbs as spares, they were all stock and very dirty with varnish in them. I took them completely apart and cleaned everything out good. The bike now runs clean all the way through the power band, I left the stock jetting in to start fresh, and it does need a little more main jet.

My thoughts on the first set of carbs. Someone had been into them before and installed bigger main jets, I think that was fine, but they also drilled the holes in the bottoms of the slides to make them open quicker and possibly installed lighter springs. I personally don't care to do this on most carbs, and I feel that the drilled slides coupled with what feels to be little lighter springs allowed the slides to open too much in the mid range rpm and going rich for a moment because of the "power valve" needle possibly raising. One solution might be to "shim" the needle down solid so that it cannot move thus making it a more standard setup.

POPS 911 · « **Reply #45 on:** July 05, 2015, 08:09:37 AM »

I have a like new set of 1980-83 NIGHTHAWK DOHC 750 carbs 34mm that I got on ebay for $100 with all the hoses and gas tank vaccum valve = I'M going to try and put them KEIHINS on one of my 1978 750 Hondamatics WITH THE NEW SOFT EARLY 69-75 BOOTS that I have, we check the spread with a set of 900cc off the CB900f and the only difference was that the 900cc are 36mm. If I can get the CV 34's on the 750a to fit and run you would have no problems tuning these simple CV KEHINS CARBS = my CM400cc Hondamatic has the same style carbs as the DOHC 750-900- and GOLD WING CARBS W/ smaller pressed in fuel jet on 400CCA [ #20 ] with the 750NH-900F having a [ # 26 ] pressed in fuel jet. Tune by shimming the needle [ two minute job], putting in a larger main jet, jet drill the idle jet up 2, run pods, 4-1 or 2-2 open header , run that L down the 1320' to 7000 rpm shift light a glowing and shift into D = maybe a 750 can run up to 9000rpm before shifting into D

?? The CB900F DOHC had much lighter springs [ shorter ] and I have found different springs in the carbs KEIHIN CV'S that I have bought.

TurboD · « **Reply #46 on:** July 05, 2015, 04:52:53 PM »

I'm not out of the woods yet, my rich midrange woes are back after I richened the main jets. Again at roughly 5,000 to almost 7,000, it goes rich and misses slightly.

The needles are non adjustable, I'm sure that if I could lower them the problem would clear as it is ever so slight. My baffle is somewhat restrictive and I am hoping that if I remove it the engine will lean a little.

Pops I have a set of 650 Nighthawk 32mm and 750 34mm Nighthawk carbs, I do think that with new soft boots the 650 32mm carbs could be made to go on. The intake spigots on the 650"s are the same size as the 750 Seca carbs. The outer two carbs are nearly the same spacing as the 750 Round tops , but the inner two are about 1/4 farther apart.

Pops My 750 auto will pull good and strong with the 750 Seca carbs to right at 11,000 on the tach, its likely actually around running 10,500.

The first picture compares 750 Seca (top) to the 650 Nighthawk.
Second picture compares 650 Nighthawk and the 750 Nighthawk, same spacing but the 750 spigots are larger.
Third picture compares 650 Nighhawk (top) to the CB750 Roundtop.
Forth picture shows carbs (left to right) 750 Automatic, 77-78 CB750F, 74 Roundtop, 83 750 Seca, 650 Nighthawk, 750 Nighthawk, 85 GPZ900 900 Ninja.

scottly · « **Reply #47 on:** July 05, 2015, 10:07:34 PM »

Quote from: TurboD on July 05, 2015, 04:52:53 PM

I'm not out of the woods yet, my rich midrange woes are back after I richened the main jets. Again at roughly 5,000 to almost 7,000, it goes rich and misses slightly.

Do a plug-chop when the motor is missing. Too rich usually doesn't usually cause a miss, unless it's so rich it soots the plugs.

TurboD · « **Reply #48 on:** July 06, 2015, 04:52:16 AM »

Yes and I have been reading the plugs for input. I am sure they are lean. They always look like new, lean with hardly any coloring. That's why I have added more main jet with two different sets of carbs, each time The engine runs stronger everywhere except this narrow rpm range in the middle. With the smaller jets it will run through this area with out miss, but you can feel that it is clearly flat and not as fast everywhere else.

I can run it through the entire rpm range from idle to redline at any throttle setting just short of full throttle and not have a problem, it only occurs at 95% or more throttle. If I take off hard (full throttle)and slightly roll back off the throttle when hit hits this range it will make it through with little or no missing.

I think to cure this problem with these carbs I need the ability to lower the needles or needles for a leaner mix in the middle.

scottly · « **Reply #49 on:** July 06, 2015, 10:03:33 PM »

Quote from: TurboD on July 06, 2015, 04:52:16 AM

it only occurs at 95% or more throttle.

That is main jet territory more than needle. Your main jets are still too small, IMHO. What are you running at the carb entries? Stacks, pods?

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Author Topic: CB750A Barn Build (Read 17027 times)